A bearing is defined as a means of positioning one part with respect to another in such a way that a relative motion is possible. This relative motion and its type are dictated by the requirements of the arrangement of which the bearing will be a part. Bearings are designed by determining the mechanical functions which the bearing must perform, the requirements on life and reliability and the ambient conditions, including temperature, corrosive atmospheres, and vibration.
The two major classes of bearings are: (1) sliding bearings, in which the bearing elements are usually separated by a film of lubricant (e.g., oil or grease) and in which a sliding motion is the predominant element, and (2) rolling-element bearings of which ball bearings, roller bearings, and needle bearings are examples.
Slider-type bearings include all varieties of bearings in which the primary motion is a sliding of one surface over another. Accordingly, all types of journal or sleeve bearings which are used to position a shaft or movable part in a radial direction are slider-type bearings. Furthermore, all types of thrust bearings, which are used in general to prevent movement of a rotating shaft in an axial direction and as guides for linear motion of various types, are also slider-type bearings. Thrust bearings vary widely in design as well, ranging from simple, flat thrust collars to complex tapered-end and pivoted-shoe (i.e., Kingsbury) bearings. Some journal and thrust bearings are designed to operate with a lubricant supply under sufficient external pressure so that the load is carried by this pressure rather than by the hydrodynamic forces generated by the sliding motion. Other bearings move slowly enough, or intermittently, or under light enough loads so that separation by a film of lubricant is not necessary for satisfactory performance and life. In this case, the surfaces are allowed to rub on each other with only the boundary-lubrication properties of the lubricant preventing seizure and wear.
Rolling-element bearings include ball bearings, roller bearings, and needle bearings. Both ball and roller bearings are made and designed to carry either radial or thrust loading or both. Needle bearings, in general, are restricted to radial loads.
Prior art lubricants vary widely from fluids of all types, including water, oil, soaps, greases and air, to solid lubricants such as graphite and molybdenum disulfide.
Among the mechanical requirements to be considered in choosing a bearing are the load to be carried and the character of the load, the surface velocity which can be tolerated by the bearing, the ability of the bearing to tolerate misalignment, the friction when starting the bearing under a load, the power consumption of the bearing, the space requirement, the type of failure that may occur, the damping capacity, and the lubrication requirements.
With regard to load, the load-carrying capacity of a slider-type bearing is very much a function of speed and of lubricant viscosity. Starting under load can cause damage in slider-type bearings due to a shearing failure during that period before the lubricant film is formed and a lowering of the friction at the rubbing interface takes place. In many cases the possibility of shearing failure limits the load which can be carried.
As to speed, slider bearings are limited by the temperature rise created by high-speed shearing action in the lubricant; and, in general, high surface speed demands relatively large clearances, low-viscosity lubricants, and large lubricant flows to remove heat. In addition to these effects, turbulence has been found to occur in the slider-type-bearing lubricant films when operating at very high speeds; and this turbulence, which greatly increases at rate of temperature rise with increasing speed, places an effective upper limit on slider-bearing velocities.
With respect to misalignment, it is a desirable mechanical characteristic in many bearing applications if the bearing can tolerate misalignment, including shaft deflections arising from changes in loading during operation, thermal gradients which cause differential expansion, and inaccurate machining and line-up. Other types of misalignment, which involve adjustment during operation, in general require that the misalignment be accommodated by the bearing structure itself.
As to friction, low starting friction, particularly when starting under load, is an advantageous characteristic of a bearing. With respect to power consumption, reduced power consumption is advantageous as well.
With regard to the space requirements, slider-type bearings can occupy less space in the direction of load support (i.e., in the radial direction for journal bearings, and in the axial direction for thrust bearings), than roller bearings.
With respect to failure, slider-type bearings offer some advantages over roller-element bearings in the event of failure of the lubricant supply because sliding can often occur on the bearing metal for an appreciable period of time without serious dislocation of the position of the moving parts. This contrasts with rolling-element bearings, particularly ball bearings, which where in failure to maintain a lubricant film on the moving parts results very quickly in severe damage to the bearing and in serious dislocation of the moving parts as rolling elements are lost, shattered or flattened.
As to damping, slider-type bearings contain within their lubricant film a certain amount of inherent damping capacity represented by the time and pressure required to reduce the lubricant-film thickness. Damping is the ability to reduce the magnitude of vibration by absorbing a portion of the vibrational energy.
With regard to lubrication, slider-type bearings, in general, require relatively large amounts of lubricant (e.g., oil) in order to maintain the lubricant film between the surfaces. Unless the loads are very low, the pressure that is generated in the lubricant film to support the load also drives lubricant out of the bearing. Lubricant lost in this manner must be replenished continuously. Specially designed slider bearings may also operate on air or gas as the hydrodynamic fluid, or as the fluid supply in externally pressurized bearings.
The environmental conditions to be considered in regard to choosing bearings, includes operating and starting temperatures, and corrosion.
With respect to operating temperature, slider-type bearings can be constructed to withstand temperatures as high as the lubricant can tolerate. As to starting temperatures, the ability to start at low temperatures is often necessary for equipment that must operate outdoors, particularly in northern climes and in aircraft. Rolling-element bearings are advantageous where low starting temperatures are a requirement, since the greater area of lubricant which must be sheared during the starting of the slider-type bearing requires application of more torque to start.
With regard to corrosion, sliding-type bearings often can be designed and constructed from corrosion-resistant materials for any particular application because of the wide range of materials available.
There are also economic factors to be considered in the selection of bearings. In this respect, the principal economic factors are life and reliability, maintenance, ease of replacement and cost. Slider-type bearings, when properly designed and when operating under reasonably uniform loading, have a virtually unlimited life. However, the lubricant-supply system of a sliding bearing will require occasional attention for replacement of the lubricant, change of filter, or general cleaning.
As to cost, slider-type bearings can be produced at very small cost in mass-production quantities, but their cost can be very large, when they have to be machined in small quantities for special designs.
As indicated above, journal bearings are one type of sliding bearing. Journal bearings are classified roughly according to the method of lubricant feed to them, as (a) non-pressurized bearings, (b) pressure-fed bearings, or (c) externally pressurized bearings.
Examples of non-pressurized bearing are bushings, wick-oil bearings and oil ring bearings, the bushing being the simplest type of journal bearing. In general, these bearings are used at low speeds and moderate loads, and careful attention must be paid to the selection of the proper bearing material and shaft material to be used.
A bushing is a sleeve of bearing material in which a shaft rides. Depending upon the application, the bushings may be run dry or they may be oiled, or grease-lubricated. In general, bushings operate at such slow speeds that it is questionable whether hydrodynamic lubrication conditions prevail and a whether complete oil film separates the bushing from its shaft. Thus, boundary lubrication is an ordinary operating condition for bushings, and the selection of both the lubricant and the bearing material becomes very important. The simplest bushing may be nothing more than a hole drilled and reamed in the structure of the machine. This may be satisfactory in a cast-iron or aluminum housing where the loads and speeds are nominal. However, the bearing material is thereby restricted to a material selected primarily on the basis of its structural and machining properties.
Wick-oil bearings receive a continuous supply of lubricant from a wick which is saturated with oil. These bearings are usually used for shafts less than one inch in diameter and running at speeds of 3,600 rpm or less. When properly designed, wick-oil bearings appear to operate under very nearly hydrodynamic conditions. The wick rubbing on the shaft's surface through a window in the top of the bearings supplies oil to the surface. Oil leaving the bearing is normally retained by some form of seal and returned by gravity to the wick. Thus, the wick provides a miniature oil-circulation system for the bearing. It also serves a secondary function of filtering the oil supply to the bearing.
Oil-ring bearings are generally used for horizontal shafts. The bearing receives its lubricant through a window at the top of the bearing through which the oil ring contacts the shaft. The oil ring is considerably larger in diameter than the shaft and passes below the bearing, dipping into a reservoir or oil beneath the shaft. The oil-ring is rotated by contact with the rotating shaft and transfers the oil picked up from the reservoir to the shaft at the top of its cycle. Spreader grooves are used to distribute the oil from the window along the length of the bearing.
Pressure-fed bearings have lubricant (i.e., oil) which is fed under pressure. A pressure-fed bearing system includes a storage tank, a pump, either a full flow or bypass-type filter or centrifuge, a cooler, a pressure regulator, a temperature regulator, supply lines to the bearings, and return lines from the bearings (which drain the lubricant from the bearings back to the tank). This type of bearing is normally used in large fixed insulations, such as power houses, and other equipment where reliability is particularly important. Because of the provision for cooling, the pressure-fed bearing is advantageous for use with high speed machinery where the heat-rejection rate is too large for normal static cooling.
There are a number of designs for pressure-fed bearings which are commonly used. Variations lie in the shape and location of the lubricant grooves and in the shape of the bearing bore. In most instances, the bearing is split into at least two halves, primarily for ease of assembly into various parts of a machine such as a turbine. The types of pressure-fed bearings include circumferential-groove, cylindrical, cylindrical overshoot, pressure, multiple groove, elliptical, elliptical overshot, three-low, pivoted-shoe, nutcracker and partial.
Externally pressurized bearings, such as pocket bearings and hydrostatic bearings, depend upon lubricant (i.e., oil) pressure from an external pressure source to support the bearing load. This differs from hydrodynamic bearings, which depend upon lubricant pressures generated in the lubricant film to support the load.
As indicated above, thrust bearings are a second type of sliding bearing. The types of thrust bearings include low-speed bearings, which largely depend upon boundary lubrications, types which operate on hydrodynamic principals and externally pressurized thrust bearings.
The flat-land type thrust bearing is comprised of a runner or collar on the shaft which bears against a flat stationary bearing surface. Ordinarily, lubricant (i.e., oil) is fed to the center of the bearing and out over the bearing surface in six to eight lubricant grooves. This type of bearing is useful only for low loads, in the range of 50 to 100 psi maximum. Bearing wear, overheating, and failure can result from imposition of loads which are too high.
The tapered-land thrust bearing is a modified grooved flat-land bearing. The flat surface between grooves is cut away in order to provide a taper in the direction of shaft motion. The tapered-land thrust bearing has high low-carrying capacity in only one direction, unlike the flat-land bearing which will run equally well in either direction. In the forward direction, the tapered-land bearing can carry loads as high as 500 to 1,000 psi.
The pivoted-shoe thrust bearing is commonly known as the Kingsbury bearing. The stationary bearing member is split into three or more shoes, each of which is then pivoted at its center. Because each shoe adjusts to give an optimum lubricant-film taper, a pivoted-shoe thrust bearing can carry high loads over a wide range of speeds and when running in either direction.
A tapered-land thrust bearing can carry loads as high as the pivoted-shoe design, but only at the design speed. On the other hand, the pivoted-shoe bearing is inherently more costly to manufacture and requires careful attention to the lubricant supply. Spring-supported flexible-plate thrust bearings are machined to very close tolerances as a flat plate and is then supported on nests of pre-loaded coil springs. Both the bearing and the runner normally are grooved in order to provide good oil distribution.
A step thrust bearing has a lubricant-film thickness which is reduced sharply at a dam or step. The step thrust bearing is particularly useful as a small thrust bearing where the very slight depression of the step can be produced either by a coining process or by etching with acid.
A pocket thrust bearing provides the advantages of the externally pressurized bearing without the disadvantage of maintaining an external high-pressure pump. Lubricant (i.e., oil) from the lubricant groove is carried over the pumping end section and into the large pocket area. Since the lubricant carried is restricted from leaking out around the boundaries of the pocket, a lubricant pressure sufficient to carry the imposed load is generated by hydrodynamic principles.
Bearings can also be classified by the type of material used for construction. The classes of bearings include plain bronze, sintered, nylon, polytetrafluoroethylene (i.e., PTFE or Teflon.RTM.), molded-fabric, hardwood, cast iron, steel, and carbon-graphite.
Bronze bearings are comprised of alloys which range from those containing aluminum for hardness to those containing large quantities of lead for softness and lubricity. Grease or oil are used as lubricants. Bearings constructed of bronze are most suitable for slow or medium motion under loaded conditions. Typical uses of bronze bearings are for heavy-machinery service, such as in the manufacture of press cranks, sleeves, electric motors, engine piston pins, and automotive kingpins.
Sintered bearings are comprised of metal powder, with or without graphite, compressed to the desired form, and sintered at high temperatures to develop strength. The metal usually used includes bronze, lead-bronze, iron, and iron-copper. Grease, or preferably oil, are employed as lubricants. Typical uses of sintered bearings include electric motors, generators, pumps, compressors, transmission components, and construction and textile machinery.
Nylon bearings are comprised of a polyamide resin. Nylon bearings can be lubricated with water or oil, or can be run dry. However, when nylon bearings are run dry the loads and speeds are limited. The typical applications for nylon bearings are food, bakery, textile and dairy machinery, automotive kingpins and steering linkages.
Teflon.RTM. bearings are comprised of polytetrafluoroethylene (PTFE). Under mild conditions Teflon.RTM. does not need a lubricant. However, oil or water can be used as lubricants where the loads and speeds are greater. The applications for Teflon.RTM. bearings are the same as those for nylon bearings, as discussed above.
Molded-fabric bearings are laminates composed of a fabric, such as cotton, impregnated with a phenolic or similar resin and molded under heat and pressure. They can be lubricated by oil, grease, or water, provided operating conditions are not severe. If the operating temperature of a molded-fabric bearing can be kept within reasonable limits (i.e., below 150.degree. F.), by means of air, water or oil cooling and lubrication, it can withstand heavy usage. Typical applications include the operations of dairy equipment, food and textile machinery, mill roll necks, and pumps.
Hardwood bearings are comprised of woods such as lignum vitae, oak or elm. Bearings comprised of lignum vitae are self-lubricating under medium speeds and low loads, or under high speeds and light loads. However, for heavy loads of up to 2,000 psi and speeds of up to 200 rpm, water lubrication is desirable. Hardwood bearings of other woods usually require oil lubrication. Hardwood bearings find typical uses in textile equipment, food machinery conveyors, marine propeller shafts, and mill roll necks.
Bearings made of cast iron are only suitable for low to moderate loads.
Cast iron bearings are lubricated with oil or grease. Suitable applications include water pumps and simple hand-operated machinery. One major problem with cast iron bearings is the formation of rust.
Bearings made of steel are typically used in applications such as drill guides in jigs and fixtures, and grinder spindles. Steel bearings are lubricated with oil.
Carbon-graphite bearings are used where temperatures are high, where lubrication is impractical or impossible, and where corrosion or chemical action rules out other materials. Accordingly, carbon-graphite bearings are normally used where bearings must run dry or submerged in a fluid. Typical uses of carbon-graphite bearings include electric motors, conveyors, and submerged mechanisms. One problem with carbon-graphite bearings is that they are brittle and subject to cracking.
Other prior art materials include those sold under the marks METALINE, MICROPOLY, and GENR. METALINE is comprised of a bronze or a phenolic substrate having recesses plugged with a non-fibrous graphite material. The non-fibrous graphite material functions solely as a lubricant for the bronze or the phenolic substrate. Furthermore, the non-fibrous graphite material will pulverize or fall out under large loads, thereby preventing this material from functioning as a structural component of the bearing arrangement. Therefore, the non-fibrous graphite material of METALINE is not suitable as a load bearing material. In addition, the phenolic substrate is suitable for continuous operation only at temperatures up to 248.degree. F. MICROPOLY is comprised of a bronze substrate having recesses filled with a nylon. The nylon is not a polyimide, consequently, it will creep and distort under load. Accordingly, this material is also deficient as a load bearing material. GENR is a relatively soft material having low flexural strength. GENR is typically used in jet engines as the bearings for the variable stator vanes. This material is made with a braid of a stretch-broken carbon yarn and thus lacks the mechanical properties necessary to resist large impacts and the wear properties necessary for use in bearing applications.
Among the drawbacks to the bearing materials discussed above is that there are some applications wherein the bearings may be difficult to access or time consuming to lubricate. In the case of oil and grease lubricants, there are also environmental hazards and high disposal costs. Moreover, the lubricant itself may be costly. In situations where equipment must be shut down in order to lubricate bearings, the cost of downtime can be very high. Other drawbacks to the bearing materials discussed above is that they may not be well suited to applications where the environment or rubbing surface temperatures are high (e.g., 500.degree. F.), where loads (i.e., pressure) may exceed 1000 psi, where speeds may exceed 100 rpm, and where there are high impact forces.
An alternative to the foregoing materials is a polymer composite material having a thermoset polyimide and carbon fibers. One such polymer composite material is derived from a sheet molding compound (SMC). SMCs are comprised of a thermosetting resin matrix, reinforcing fibers and sometimes other modifiers, in sheet form. The possible resin matrices include polyesters, epoxies, vinyl esters, phenolics and the like. The reinforcing materials may take the form of graphite fibers (a.k.a. carbon fibers), glass fibers, ceramic fibers, or combinations of the preceding, together with non-reinforcing fillers, depending upon the desired properties.
In particular, the polyimide sheet molding compound disclosed in U.S. Pat. No. 5,126,085, which is incorporated herein by reference, is particularly well suited as a material for bearings and bearing surfaces. This material is known commercially as WearComp.RTM.. It is comprised of a PMR-15 polyimide resin (a high temperature thermosetting resin) and 0.125 inch to 2.0 inch graphite fibers which are used as reinforcing materials. This polyimide SMC yields cross-linked products having a room-temperature tensile strength in the order of from about 45,000-50,000 psi and a modulus of about 5 Msi. The cross-linked polyimide products display a useful life of over 2,000 hours at temperatures in excess of 550.degree. F., and up to 1,000 hours at 600.degree. F. In addition, they provide char yields on the order of 70%, providing an insulating barrier against the spread of flame. The characteristics of the polyimide composite material includes low friction, high wear resistance, low creep, good dimensional stability, superior impact resistance, and self-lubrication. Consequently, it lends itself to a variety of applications including the fabrication of self-lubricating parts, and for high temperature uses such as engine parts and aircraft brakes.
Table I set forth below provides the mechanical properties for HyComp 310,.TM. which is one of the WearComp.RTM. composites.
TABLE I ______________________________________ Typical Mechanical Properties of HyComb 310 .TM. ______________________________________ Tensile Strength Ksi 73.degree. F. 49 500.degree. F. 44 Flexural Strength Ksi 73.degree. F. 80 500.degree. F. 75 Flexural Modulus Msi 73.degree. F. 5.6 500.degree. F. 5.2 Compressive Strength Ksi 73.degree. F. 103 600.degree. F. 63 Impact, Notched Izod 12 ft. lb/in. ______________________________________
Although the WearComp.RTM. composites have the foregoing notable characteristics, they also have a relatively high material cost as compared to other materials. Accordingly, it would be advantageous to provide the bearing or wear surfaces of a bearing component with the qualities of a WearComp.RTM. composite while using as little composite as possible. This substantially reduces the material cost of the bearing component, particularly where the bearing component is relatively large.
As indicated above, a typical bearing arrangement requires a lubricant, such as oil or grease for proper operation. Accordingly, these bearing arrangements will require periodic maintenance to replenish or replace the lubricant. In some cases, when a particular piece of machinery is shut down for servicing, consequently requiring shutting down of a line of machines, the cost of downtime can be as much as $650-$700 per minute.
One situation in which a bearing arrangement is used under harsh conditions (e.g., large loads, high impacts and high temperatures), in which the bearing components are relatively large, and the downtime costs can be very high, is in metal processing factories, such as steel or aluminum mills. With regard to mills producing rolled sheet steel, coated sheet steel and mill products (e.g., tin plate and chromium-coated steels), bearing arrangements are used in equipment such scale breakers, roughing stands, finishing stands, vertical edgers, coilers, pay-off reels, steering rolls, shears, recoilers, and welders. It will be appreciated that while the present invention is particularly well-suited for bearing arrangements in a steel or aluminum mill, and is described with particular reference thereto, it also finds use in components used under similarly harsh conditions.